1. Field of the Invention
The present invention relates to a laminated glazing material with properties of acoustic insulation and mechanical strength.
2. Discussion of the Background
Laminated glazing materials are generally designed for use in vehicles and buildings to reduce the audibility of external noises within the interior. Laminated glazing materials also have major advantages in terms of their mechanical strength. In fact, in the event of an impact, an intermediate layer of a laminated glazing material advantageously permits part of the impact energy to be absorbed by viscous dissipation before the glass breaks. The function of the intermediate layer is also extremely important since it ensures that the structure will be largely preserved if the glass is completely cracked by virtue of adhesion of glass fragments to the film. Thus, the intermediate layer prevents projection of glass splinters, and consequent injury to persons in the vicinity of the broken glass.
Laminated glazing materials are commonly constructed using polyvinyl butyral (PVB) due to the mechanical performances of this material. Nevertheless, PVB has poor acoustic characteristics. Accordingly, special resins are sometimes preferred for their improved acoustic performances.
The choice of resin for laminated glass constitutes an important criterion for sound insulation of the glazing material. This choice can be made using a method for determining the critical frequency of the laminated glass and comparing the result with the critical frequency of a glass bar. Such a method is described in European Patent EP-B-0 100 701. In this patent a resin is considered to be suitable if a bar of 9 cm length and 3 cm width made of laminated glass including two glass sheets of 4 mm thickness joined by a 2 mm layer of the resin has a critical frequency which differs at most by 35% from that of a glass bar of 4 mm thickness having the same length and the same width.
However, resins with high acoustic performances do not always have the mechanical properties necessary for the conditions in which the laminated glass will be utilized.
For the purpose of combining both acoustic and mechanical properties, European Patent EP-B-0 783 420 proposes the combination of a polyvinyl butyral film with a resin film having acoustic performances. The combination of two separate films, however, leads to higher product costs and to an increase in the cost of producing the glazing material. In fact, the combination of multiple plies of material for the intermediate layer does not allow each material to be recycled individually from the surplus generally produced at the end of the manufacturing line, whereas the recycling operation can be readily employed to optimize production profitability when the intermediate layer is constructed of a single ply.